We have proposed an analytic formulation of transport in isolated nephron segments of the rabbit. Preliminary results indicate that a rectangular geometry for intercellular transport channels must be modified to insure efficient transport of isotonic absorbate independent of trans-tubular hydrostatic pressure differences. Electron microscopic studies of cellular and intercellular morphology are in progress. The technique of in vitro microperfusion of isolated nephron segments presently is being employed to determine the relationship between hydrostatic pressure and transport rate in proximal nephron segments. Studies on thin descending limb of Henle will be submitted for publication in the near future.